All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is undesirable to wear pendants containing any of the Names of Allah or His Attributes or any person's name that contain one of these Names or Attributes, such as ‘Abdullah or Abdur-Rahmaan.
The reason for this is that one may enter the bathroom with these things or lay on them and this is considered as ill treatment to these things which contain one of the Names or Attributes of Allah.
In addition, writing the Name of Allah on jewelry is a kind of imitation of Christians and Jews who wear pendants with the cross or the star of David, and their imitation is unlawful.
As for writing a letter of the name of the person on the pendant or his entire name, then there is no harm in that as long as this name does not contain one of the Names or Attributes of Allah.
Allah knows best.